1. Field of the Invention
The present invention relates to a backlight unit and a liquid crystal display device having the same, in which a viewing angle of the liquid crystal display device may be enlarged or changed and it is advantageous to form the liquid crystal display device in a thin shape.
2. Discussion of the Related Art
As the information-oriented age has arrived, the field of display which visually displays an electrical information signal has rapidly developed. Accordingly, various flat display devices with excellent features such as thinness, light weight, and low power consumption have been continuously developed.
As representative examples of the flat display devices, there are an LCD (Liquid Crystal Display device), a PDP (Plasma Display Panel device), an FED (Field Emission Display device), an ELD (Electro Luminescence Display device), an EWD (Electro-Wetting Display device), an OLED (Organic Light Emitting Display device), etc.
Such flat display devices commonly include a flat display panel which is essential to display an image. The flat display panel has a structure in which a pair of substrates is bonded opposite to each other while an inherently luminous or polarizing material is interposed therebetween.
Among others, the liquid crystal display device is a device which displays the image using optical anisotropy and polarization of liquid crystals. That is, the liquid crystal display device includes a liquid crystal panel comprising the pair of substrates and a liquid crystal layer interposed therebetween. The liquid crystal layer comprises the liquid crystals which have elongated shapes and are oriented in a predetermined initial direction. The liquid crystal panel forms an electric field to deform the directions of the liquid crystals for each pixel area in order to adjust a light transmittance of each pixel area. Therefore, the LCD displays the image.
Since the liquid crystal display device is not a device which includes an inherently luminous material to emit light by itself, the liquid crystal display device generally includes a BLU (Backlight Unit) which irradiates light onto the liquid crystal panel.
FIG. 1A is a cross-sectional view illustrating a general BLU (Backlight Unit), FIG. 1B is a cross-sectional view illustrating a light guide plate of the general BLU illustrated in FIG. 1A, and FIG. 1C is an image illustrating a dot pattern of the light guide plate illustrated in FIG. 1B.
As shown in FIG. 1A, the backlight unit 10 is disposed below the liquid crystal panel 20 and irradiates light toward the liquid crystal panel 20.
The backlight unit 10 includes a light source 11, a light guide plate 12 which converts light emitted from the light source 11 into a surface light source, a diffusion sheet 13 and a prism sheet 14, and a reflective sheet 15.
The light source 11 is disposed on at least one side of the light guide plate 12 and emits light to the light guide plate 12.
The light guide plate 12 guides incident light supplied from the light source 11, and emits light upon a light emitting surface (an upper surface of the light guide plate 12 illustrated in FIG. 1A) facing the liquid crystal panel 20. The light emitted from the light guide plate 12 has an incident angle which is equal to or less than a critical angle.
The diffusion sheet 13 and the prism sheet 14 are disposed above the light guide plate 12. The diffusion sheet 13 diffuses light emitted from the light emitting surface of the light guide plate 12, and the prism sheet 14 concentrates light.
The reflective sheet 15 is disposed below the light guide plate 12 and reflects light toward the light guide plate 12.
As shown in FIG. 1B, the light guide plate 12 includes a plurality of dot patterns 12b formed on an opposite surface (shown as a lower surface in FIG. 1B) of the light emitting surface 12a. Since light is diffused and scattered on the plurality of dot patterns 12b, the incident angle of light reached the light emitting surface 12a may be changed and thus more light has the incident angle being equal to or less than a critical angle. Therefore, the plurality of dot patterns 12b may allow light restricted within the light guide plate 12 to be reduced, resulting in an enhancement in luminance of the liquid crystal display device.
Each of the dot patterns 12b is generally produced on the light guide plate 12 using a stamper processed in a laser dotting method. Thus, as shown in FIG. 1C, there are problems in that a burr is generated around the dot pattern 12b and light is irregularly scattered from the burr.
Accordingly, in order to prevent the burr generated around the dot pattern 12b from being viewed, the backlight unit 10 should necessarily include the diffusion sheet 13 disposed between the light guide plate 12 and the liquid crystal panel 20. For this reason, there is a limit in forming the backlight unit 10 and the liquid crystal display device having the same in a thin shape.
In addition, since light is irregularly scattered by the dot pattern 12b, light emitted from the light guide plate 12 has an irregular emission angle. Thus, it may be impossible to enlarge or change a viewing angle of the liquid crystal display device. Particularly, in a case where the liquid crystal display device including the general BLU is applied to a device requiring a wide viewing angle such as a navigation device, there is a problem in that another configuration for adjusting the emission angle of light emitted from the backlight unit 10 is further required in order to enlarge or change the viewing angle.
That is, in a case of a television or a monitor, since a viewer uses the device in a state of being located in the front of the device, the viewing angle of the liquid crystal display device applied to the television or the monitor needs to be designed so as to focus on a center in the front of the device. The viewing angle of the liquid crystal display device, particularly for security, may also be limited to focus on only a central area of the device.
On the other hand, the navigation device is not generally located in the front of the driver so as not to hinder a driver's view, and particularly, is generally fixed to a center of a front seat such that all passengers in a vehicle may view the navigation device. Thus, the viewing angle of the liquid crystal display device applied to the navigation device needs to be more widely enlarged than that applied to the television or monitor, in order to cover the sides of the device. Consequently, in order to apply the liquid crystal display including the general BLU to the navigation device, the liquid crystal display should include an optical sheet for enlarging or changing the viewing angle thereof, the liquid crystal display device has limited thinness and lightness.